Design a permanent card that cares about converted mana cost (either of itself with or of other cards.).

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Fermat

It That Humbles
Creature — Eldrazi
~ enters the battlefield with X +1/+1 counters on it.
Creatures your opponents control with converted mana cost less than or equal to ~'s power lose all abilities, have base power and toughness 1/1, and can't attack you or a planeswalker you control.The elves watched it rise from the ashen remains of their forest and dropped their bows in awe.
0/0

fluffyDeathbringer

Mystic of the Next Stratum
Creature - Human Cleric : Add one mana of any color to your mana pool. Spend this mana only to cast a spell with converted mana cost 6 or greater.Lost to mortals, her mind contemplates only the actions of the divine.
1/2

Inverness

Universal Harmony
Legendary Enchantment
At the beginning of your upkeep, if the total converted mana cost of all permanents is exactly 22, you win the game.The truth of the universe is both incalculable and undeniable.

angel666

Endril, the Mathematician
Legendary Creature — Human Wizard
At the beginning of your upkeep, if you control a permanent with converted mana cost 1, 2, 3, 4, 5, 6, 7, 8 and 9, you win the game.
2/2